The invention concerns a device for the prevention of the abuse of a dry-plate clutch for a motor vehicle with a manual gearbox, wherein the device acts jointly with a hydraulic master cylinder and a related slave cylinder in the operation of the clutch.
The use of a device of the above-mentioned type prevents the transfer of an excessive torque gradient (torque change per time unit) to the transmission due to a too rapid release of the clutch pedal during the engagement of the clutch. Such abuse of the clutch is especially risky when starting in, first gear or in reverse gear, as well as when moving through the lowest gears when driving forwards. The sudden forces acting on the transmission during such a rapid engaging of the clutch often result in damage to the differential gears, the crown wheel and the pinion of the rear axle transmission unit, the drive shaft and the gearbox, but damage has also been found in engines due to frequent abuse of the clutch. There are previously known devices for counteracting the abuse of the clutch, where the devices are not included in the hydraulic clutch system, but comprise levers and rods which are connected to the clutch pedal or the clutch master cylinder's piston rod or the clutch disengagement arm, and are effective shock absorber. These mechanical systems must be manufactured with fine tolerances, are difficult to instal, expensive and dampen the clutch movement every time it is engaged, even when the clutch pedal is released in the correct manner.
Another known system for counteracting the abuse of the clutch works hydraulically via a choke device with a non-return valve installed in a bypass pipe. When the clutch is engaged the non-return valve closes, thus causing all the hydraulic fluid to flow via the choke device, thereby providing a damping during the entire return movement of the clutch pedal. In this case too a damping takes place each time the clutch is engaged, irrespective of whether the clutch pedal is moved rapidly or slowly. Since the damping takes place during the entire return movement, a rapid gear change is virtually impossible, a fact which is particularly critical and awkward when changing gear at low temperatures since the damping increases and the clutch can slip.
For this reason, in another embodiment of this system an electromagnetically controlled valve is incorporated which engages the device only when the clutch is used in connection with first gear and reverse gear. Thus there is no protection against abuse of the clutch in the other gears.
From DE1600184, DE3031262 and FI842051 there are known devices of the above-mentioned type where a valve ensures that a pressure-based clutch can be engaged rapidly until a predetermined pressure has built up in the hydraulic fluid, whereupon a delay occurs. Apart from this no consideration is given to whether the clutch is operated rapidly or slowly. Thus this solution is not suitable for spring-based dry-plate clutches.
The object of the invention is to provide a device of the type described in the introduction which is not encumbered by the above-mentioned disadvantages.